The nickel hydroxide widely used as a positive electrode active material in alkaline batteries typically undergoes an oxidation-reduction reaction between β-type nickel hydroxide (also referred to as β-Ni(OH)2 in which the valence of Ni is 2) and β-type nickel oxyhydroxide (also referred to as β-NiOOH in which the valence of Ni is 3), and discharging and charging of the battery takes place as a result thereof. This oxidation-reduction reaction can be represented with the reaction formula indicated below.β-Ni(OH)2+OH−⇔β-NiOOH+H2O+e−
The oxidation-reduction reaction represented by the aforementioned reaction formula is reversible. Namely, in this reaction formula, the reaction that proceeds from left to right constitutes charging, while the reaction that proceeds from right to left constitutes discharging. In addition, this reaction is a one-electron transfer reaction, and the theoretical capacity is known to be 289 mAh/g.
In contrast, attention is currently being focused on the oxidation-reduction reaction that occurs between α-type nickel hydroxide (also referred to as α-Ni(OH)2 in which Ni has a valence of 2) and γ-type nickel oxyhydroxide (also referred to as γ-NiOOH in which Ni has a valence of 3). This reaction is a multi-electron reaction and what is more, since it is possible for the theoretical capacity of this reaction to be at least 1.5 times greater than or equal to the theoretical capacity of the aforementioned reaction, this reaction makes it possible to realize higher battery capacity.
However, α-type nickel hydroxide is comparatively unstable and thus, is easily converted to β-type nickel hydroxide in an alkaline solution.
Moreover, a reaction is also known to occur between β-type nickel hydroxide and γ-type nickel oxyhydroxide. However, the use of this reaction is not preferable due to the difficulty in allowing the oxidation-reduction reaction to proceed reversibly and the occurrence of fluctuations in volume of the nickel hydroxide attributable to differences in the structures of these nickel hydroxides.
Thus, studies have been conducted on an ca-type nickel hydroxide capable of demonstrating a high level of stability under conditions such as being placed in an aqueous alkaline solution.
The sealed alkaline zinc storage battery of Patent Document 1 uses nickel hydroxide for the positive electrode active material, and at least one type of element selected from manganese (Mn), aluminum (Al), cobalt (Co), yttrium (Y), ytterbium (Yb), erbium (Er) and gadolinium (Gd) is solid-dissolved in this nickel hydroxide.
The alkaline battery of Patent Document 2 contains an active material consisting mainly of α-type nickel hydroxide (α-Ni(OH)2), and a hydroxide and/or oxyhydroxide containing at least one element selected from erbium (Er), trillium (Tm), ytterbium (Yb) and lutetium (Lu).
The positive electrode material for an alkaline battery of Patent Document 3 is Al-substituted Ni(OH)2. This Al-substituted Ni(OH)2 is represented by the formula (AlxNi1-x)(OH)2Y (wherein, x represents a value from 0.15 to 0.3 and Y represents an anion), and is characterized by the d value of the (003) diffraction peak in an X-ray diffraction diagram being less than 8.2 Å.
Furthermore, in the positive electrode material for an alkaline battery of Patent Document 4, solid solution particles consisting mainly of nickel hydroxide are coated with cobalt oxide obtained by putting into solid solution at least one type of metal selected from alkaline earth metals and transition metals having cobalt hydroxide for the main component thereof. In the positive electrode active material for an alkaline battery of Patent Document 4, the cobalt oxide is oxidized electrochemically when a battery employing this active material is charged, and an electrically conductive network is formed by a plurality of solid solution particles, resulting in improvement of the utility factor of the active material in the battery.